Field of the Invention
The invention relates to a NO.sub.x reduction system with a device for metering in the reducing agents. The system is used in an internal combustion engine operating with excess air, in particular a diesel internal combustion engine, and it includes: an SCR catalytic converter in the exhaust system of the internal combustion engine, a reducing agent container for storing reducing agent, a metering device for the controlled delivery of the reducing agent into the exhaust system upstream of the SCR catalytic converter, and a control device with associated sensors and actuators for controlling the NO.sub.x reduction system.
While the exhaust gas of an Otto(spark-ignition) internal combustion engine operated with an air factor .lambda.=1 can be effectively cleaned with respect to the main emissions of nitrogen oxides No.sub.x, hydrocarbons HC and carbon monoxide using so-called three way catalyst, such a catalyst is of only limited suitability for the after-treatment of exhaust gases from an internal combustion engine operating with excess air (.lambda.&gt;1) (diesel internal combustion engine or Otto internal combustion engine with lean mixture). Only the hydrocarbons and the carbon monoxide are substantially abated, while the level of nitrogen oxides cannot be significantly reduced on account of the high oxygen content in the exhaust gas.
The level of nitrogen oxide emissions from internal combustion engines of this type can be reduced using the so-called selective catalytic reaction (selective catalytic reduction, SCR process). As a reducing agent, ammonia NH.sub.3 is frequently made available to the SCR process by urea hydrolysis. The urea entrained in aqueous solution is thereby used as an ammonia carrier and is converted to ammonia using a metering system upstream of a hydrolysis catalyst. The ammonia then reduces the nitrogen oxides in the actual SCR catalytic converter. Further, it is also possible to use ammonia directly as a reducing agent. No hydrolysis catalyst is then necessary.
An NO.sub.x reduction system of this type, with an SCR catalytic converter and metering of the reducing agent, requires sensors and actuators, such as temperature sensors and NO.sub.x sensors in the exhaust, upstream and downstream of the SCR catalytic converter, respectively, a metering valve, a reducing agent container with measurement of the fill level and temperature, a heating device, and a feed system (pump and lines). In order to control the NO.sub.x reduction system, special control and regulation algorithms are needed for accurate metering and for the reducing agent container, in addition to the internal combustion engine management.
The following problems result:
a) The electrical connections of all the sensors and actuators in the exhaust system and the reducing agent container require an elaborate extension to the wiring harness of the internal combustion engine. This is particularly so since these components cannot be mounted directly at the internal combustion engine or in the vicinity of the electronic control unit of the internal combustion engine. PA1 b) The long cable lengths require increased outlay in terms of electromagnetic compatibility, and in particular the sensors require separate ground lines in order to permit correct measured values in spite of heavy currents for the actuators (metering valve, feed pump and heating of the reducing agent). PA1 c) The sensors and actuators take up extra inputs and outputs at the electronic control unit of the internal combustion engine. The computer of the control unit is burdened by data specific to metering and the tank, for example valve characteristics, sensor responses and by controlling the metering, heating and feed pump. PA1 an SCR catalytic converter arranged in an exhaust system of an internal combustion engine; PA1 a reducing agent container for storing reducing agent; PA1 a metering device communicating with the reducing agent container for controlled delivery of the reducing agent into the exhaust system upstream of the SCR catalytic converter in a direction of exhaust gas flow in the exhaust system; and PA1 a control device assembly integrated in the reducing agent container for controlling the NO.sub.x reduction system, the control device assembly being a functional unit comprising a control device and associated sensors and actuators for storing, feeding, conditioning, and metering the reducing agent.